The elements of a high performance analog RF optical link have received considerable study and attention, but system performance continues to fall short of aggressive application performance goals. Considering the RE link as a whole, methods of extended range phase modulation include phase tracking loops that are limited to low bandwidth due to active electronics and closed loop delays. At the optical to electrical conversion end of the link, high power, high efficiency photodiodes have been a longstanding topic of active research. Recent reports of 19 dBm output RE power at 25 GHz from a 20 micron device demonstrate impressive performance, but exhibit RF power reduction effects in small high bandwidth devices attributable to current saturation effects. In between these parts of the link, system operation has focused on minimizing loss.
High performance Optical-RF links can exploit use of electro-optic phase modulators in a Mach-Zehnder topology and balanced high-power photo-detection. Balanced detection offers suppression of RN and even-order harmonics, and 6 dB RF gain over a single-ended approach. Typically, the RF input signal creates small optical sideband signals. The desired RF signal is generated by beating optical sidebands created by phase modulation with the optical local oscillator (LO). The residual carrier after the modulator contains little useful signal information. The residual carrier and the LO each generate current in the photodiode that contribute to RIN, shot noise and power. The large residual carrier contributes to RIN but not signal gain, thereby impairing noise performance. In traditional approaches, RF optical links are dominated by Watt-level optical signals and a bare minimum of active elements and control.
Hence, there is a need to optimize analog RF optical links.